The actual mass flow rates of flowing gases are very important process parameters in the commercial world. However, because it was not previously feasible to measure unknown gas mass flow rates over wide dynamic ranges, especially when the flow was at low mass flow rates, it was necessary to rely on measurements of volume flow rates which could be made in spite of certain deficiencies. Such deficiencies arise from the facts that the actual volumes of any quantity of gas is dependent on at least pressure and temperature.
An example of why mass as opposed to volume is the definitive parameter for measuring quantities of gas is the fact that chemical reactions take place on the basis of calculable amounts of mass, not volume, e.g., it is predeterminable masses of hydrogen and oxygen that can be combined to form a given quantity of water. Though chemical reactions can be implemented using metered volumes of gases, it is actually the masses of the gases that must be first determined and then converted to volumes of gas. So, if only metered volumes of gases are measurable instead of metered masses of gases, the measured volumes are essentially intermediate substitutes for the required mass quantities, and the measured volume quantities have to be converted to mass quantities or vice versa to ensure provision of necessary quantities of gas to carry out reactions. The conversions between volume and mass or vice versa are unavoidably prone to error due to unpredictable physical parameter fluctuations such as temperature and pressure. As another example, specific amounts of energy, such as releasable thermal energy, through burning are available from calculable masses of certain gases. Application of scientific principles establish that there are a given number of British Thermal Units (BTUs) per pound for each given type of natural gas. In spite of this well-known scientific fact as to the relationship of BTUs and the mass of natural gases, commercial sales of natural gases are made on the basis of volume measurements because it was only such measurements that were previously feasible except in certain limited situations such as when measurements of high mass flow rates of gases or liquefied gases were made.
The difficulty in measuring gas mass flow rates in substantial part arises because gases have extremely low masses. These low masses when moved in a straight line produce essentially minute momenta or when moved in a straight line in combination with angular rotation such as flowing the gases through a vibrating or rotating pipe produce very small, almost undetectable gyroscopic or Coriolis type forces. Any system for measuring gas mass flow rates that must utilize extremely high gas mass flow rates to compensate for the low masses of gases is by this requirement alone extremely limited in applicability. The practical realities are that any instrument used to measure gas mass flow rates over wide dynamic ranges must be both inherently sensitive to the minute forces produced by flowing gases, and also essentially insensitive to extraneous forces produced by phenomena other than the flow of gases to be measured. As an example, practical applications can require measuring gas mass flow rates that are on the order of one pound per day. This is on the order of 1000 times lower than typical mass flow rate measurements for liquids. All of which results in the fact that though there have been many attempts--motivated by scientific or commercial necessity--to design instruments to measure gas mass flow rates over wide dynamic ranges, these attempts have failed or the resulting designs have been substantially limited in practical utility.
An early reported attempt to design a meter asserted as being capable of measuring the flow of either liquids or gases is described by G. P. Katys in a book titled Continuous Measurement of Unsteady Flow, Pergamon Press Ltd., 1964 at pp. 54-60. This described meter, identified as being a gyroscopic flow meter, incorporates a flow tube mounted on an annular rotor that is continuously rotated. The use of the word gyroscopic to identify this meter as being of a specific type is consistent with the provided explanation for how the meter was believed to operate, which explanation was premised on generation of gyroscopic moments. An alternative description of the operation of the meter is also set out in this book in terms of generated Coriolis forces. Irrespective of the genesis of the mathematics used to describe the operation of the meter, it is disclosed there that a flow tube is connected to fixed tubing using couplings that permit continuous rotation of the flow tube with respect to the fixed tubing. This continuous rotation in combination with the passage of some mass flow rate for a fluid through the flow tube is asserted to generate forces that cause the flow tube to deflect about an axis different from the axis about which the continuous rotation occurs. Such Coriolis/gyroscopic force induced deflection of the flow tube occurs about an axis passing through bearings interconnecting the flow tube with the annular rotor, and additionally such deflection is also described as being facilitated by two flexible couplings incorporated as parts of the mounted flow tube. The flow tube is described as being a special, mechanical system including multiple interconnected linear segments at various orientations with respect to each other. Among these are six sections identified as being closed and arranged for the purpose of compensating centrifugal forces occurring in fluids passing through the flow tube. As described, the meter provides no mechanical or other mechanism for compensating radial acceleration forces arising solely from continuous rotation of the fluid filled flow tube as opposed to forces caused by fluid flow. It may have been thought that radial acceleration forces arising from rotation of the filled flow tube would be absorbed or in some fashion damped by the flexible couplings. Irrespective of intended functions for the flexible couplings, it has been understood and widely recognized, subsequent to these disclosures, that flexible couplings unavoidably respond to fluctuating fluid pressures so that the flexible couplings themselves become sources of significant forces which inherently limit, if not absolutely, preclude the making of accurate measurements of mass flow rates of even dense liquids passing therethrough, much less of those of gases with their minute momenta. Pressure induced forces produced from flexible couplings are unpredictable for every fluid pressure fluctuation causing flexure of such couplings, and, therefore, it is not possible to calibrate this meter to make repeatable or accurate measurements.
Another prior meter identified as being of the gyroscopic type is described in U.S. Pat. No. 2,831,349. According to the description for this meter, the entire sensing element, including a tube, brackets, the fluid material flowing through the tube and the measuring elements, must be balanced. Additionally, it is stated that the centrifugal components of the entire sensing element mass must be balanced about the plane of the gyroscopic spin axis and the drive axis. As described, one of the embodiments for a meter of this type has a sensing element, that includes multiple flow loop segments through which fluid is passed. These flow loop segments are all interconnected so that fluids flow through every flow loop segment. This sensing element composed of the interconnected flow loop segments is supported inside a gimbal by a pair of torque bars that mechanically interconnect the sensing element with the gimbal, and which in combination with a pair of bellows are supposed to permit deflection of the sensing element under the influence of a generated gyroscopic couple. The pair of bellows are integrally incorporated as part of the interconnected flow loop segments so fluids pass through them. Since the momenta for gas mass flow rates are truly minute in magnitude, the resulting forces from combining rotation and gas momenta would be substantively eclipsed, both by pressure induced forces arising from the bellows and the spring forces of the torque bars which resist deflection no matter how slight the spring forces. In fact, even if the spring forces arising from the torque bars were theoretically so slight as to permit measurable deflections in response to gas mass flow rates, the bellows would unavoidably and unpredictably introduce variable pressure induced forces that would exceed the magnitude of gas mass flow rate induced forces and thus prohibit prediction of any meaningful relationship between the mass flow rates of flowing gases and the resulting deflection angles.
The need for accurately measuring gas mass flow rates over wide dynamic ranges has long been recognized, but the technical difficulties that must be addressed to make such measurements have not been previously overcome.